1. Field of the Invention
The invention relates to the method of producing zirconium or hafnium oxychloride free from phosphorus and other impurities. More specifically, it deals with controlling the total acidity of the liquor in which the zirconium or hafnium tetrachloride has been dissolved.
2. Description of the Prior Art
A typical procedure for preparing zirconium oxychloride is to dissolve zirconium tetrachloride in water in proportions so as to form a hot saturated solution containing approximately 1.5 lbs. per gallon of zirconium or approximately 2 M in zirconium and having a free acid of 4-6 N and a total acid of 8 N or higher. Upon cooling, zirconium oxychloride crystals form. Zirconium oxychloride is known to have a minimum solubility in 8 N hydrochloric acid.
Zirconium oxychloride is customarily prepared in a manner so as to produce a maximum yield of crystals. These conditions of high metal concentration and a total acid of approximately 8 N are not satisfactory for phosphorus removal but are, however, adequate for removal of most impurities such as iron, titanium, etc.
As stated above, crystallization of zirconium oxychloride by the known procedures can produce a product with a low level of most impurities. If phosphorus is present in the starting material (for example, zirconium tetrachloride) in any appreciable amount (such as 1000 ppm P/Zr), it will not be significantly removed by crystallization of the zirconium oxychloride because of the formation of an insoluble zirconium phosphate compound.
This procedure of obtaining the maximum production of crystals is more well described in U.S. Pat. No. 3,032,388 issued to McCord etal wherein it can be seen that a total acidity of about 8 N is strived for.
The difference between total acidity and free acid can be best understood by referring to the formation of zirconium oxychloride from zirconium tetrachloride. The zirconium tetrachloride is dissolved in a solution of water and hydrochloric acid. In the solution the free acid is comprised of hydrochloric acid which is added to the solution plus the hydrochloric acid that forms in the reaction of zirconium tetrachloride with water to produce the zirconium oxychloride and 2 moles of hydrochloric acid for every mole of zirconium tetrachloride. The total acidity is the sum total of free acid plus the zirconium oxychloride. It is therefore evident that total acidity is much higher than the free acid.
In the McCord, etal, reference U.S. Pat. No. 3,032,388 there is mentioned various additions of 5 N hydrochloric acid and 6 N hydrochloric acid which would define part of the free acid but would also indicate that the normality of the total acidity would be considerably higher. In these high concentrations, phosphorus will not be dissolved in the mother liquor but will precipitate with the zirconium oxychloride. One can see from Table II of this patent where they have a normality of total acid of 9.24 that there is no concern for the phosphorus. Some other prior art processes for forming zirconium oxychloride are U.S. Pat No. 3,057,678 issued to Clearfield and British Pat. No. 465,605.
When phosphorus is not removed from the oxychloride, certain procedures are required at a later time in processing the metal to remove the phosphorus. This is necessary for the product metal to have satisfactory ductility and good corrosion resistance in high pressure steam.